For popularization of electric cars, high expectations are placed on an air battery, which has far higher energy density than a lithium-ion battery. The air battery uses oxygen in the air as a positive electrode active material.
A lithium-air battery is known which uses, as a negative electrode active material, metal lithium, an alloy of which the main component is lithium, or a compound of which the main component is lithium. Depending on the type of electrolyte, lithium-air batteries can be roughly categorized into two types: one using an aqueous electrolyte (solution-based electrolyte, aqueous electrolyte solution) and the other using a non-aqueous electrolyte. The lithium-air battery using the non-aqueous electrolyte is mainly researched and developed since the techniques for lithium-ion batteries except for those for an air electrode can be utilized for the lithium-air battery using the non-aqueous electrolyte.
At the same time, lithium-air batteries using aqueous electrolyte is also being researched and developed, albeit still only a few in number. A lithium-air battery using an aqueous electrolyte has advantages over a lithium-air battery using a non-aqueous electrolyte in that the lithium-air battery using the aqueous electrolyte is not affected by moisture in the air and uses a low-cost and incombustible electrolyte. However, the metal lithium as the negative electrode active material reacts with oxygen and water when coming into direct contact therewith. To avoid this, in a lithium-air battery using an aqueous electrolyte, a solid electrolyte having lithium-ion conductivity is used as a protection layer to protect the metal lithium from the atmosphere and solutions.
A NASICON-type Li1+xAxTi2−x(PO4)3 lithium conducting solid electrolyte (hereafter, referred to as NASICON-type solid electrolyte) is known as such a solid electrolyte (Non-patent Document 1 and the like).
The NASICON-type solid electrolyte has low sensitivity to moisture, can be prepared in open air, and is stable while being in contact with a LiCl solution. Moreover, the NASICON-type solid electrolyte has favorable lithium-ion conductivity.
Non-patent Document 1 describes a NASICON-type solid electrolyte having a composition of Li1.4Al0.4Ge0.2Ti1.4(PO4)3.
However, since many pores are open on the surface of this solid electrolyte, the solid electrolyte has a low relative density of 91.2% at maximum, and a substance may permeate the solid electrolyte through these pores. Accordingly, when the solid electrolyte is used in a portion in contact with moisture such as a separator used to separate, for example, a lithium negative electrode and an aqueous electrolyte solution in a lithium-air battery using the aqueous electrolyte solution, water may permeate the solid electrolyte.
In order to prevent the permeation of water, filling the pores of the solid electrolyte with an epoxy resin or the like is considered. However, in this case, there arises unfavorable problems that it is necessary to employ an additional step of filling the pores of the solid electrolyte, and the filled epoxy resin makes the lithium-ion conductivity lower than that in the state in which the pores are not filled.
Secondarily, Non-patent Document 1 describes the NASICON-type solid electrolyte having the composition of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 as described above.
In the case in which the solid electrolyte is actually employed to manufacture the air battery and is used for a long period in a moving body such as an automobile, the strength of the solid electrolyte becomes a major issue. A three-point bending strength is one index of strength. However, the three-point bending strength of publicly-known solid electrolytes with high lithium-ion conductivity is insufficient as a solid electrolyte to be used for the aforementioned purpose.
At the same time, solid electrolytes with improved strength are sold and some of them have strength of about 100 N/mm2 which is a practically usable level. However, such solid electrolytes have lithium-ion conductivity of about 1.0×10−4 S/cm and decrease output from the level sufficient as an air battery.
The aforementioned Li1.4Al0.4Ge0.2Ti1.4(PO4)3 described in Non-patent Document 1 has the favorable lithium-ion conductivity and is empirically known to improve the strength effectively if an increased amount of Al is added thereto.
However, when an atomic ratio (ratio of number of atoms) of Al in the NASICON-type solid electrolyte having such a composition exceeds 0.4, Al atoms not packed in the crystal structure deposit as impurities, and the atomic ratio of Al cannot be increased. Accordingly, the NASICON-type solid electrolyte has a problem that the strength and the lithium-ion conductivity of the solid electrolyte itself cannot be improved.